Hi tomventiegemail, I can't really understand your message...
The fastest reaction I know propagates at 10100m/s (...and is inadequate here) so in microseconds it propagates to centimetres. You plan to make a reaction within a thin film, starting at all the surface simultaneously by some other means like a flash of light, is that it?
If you detail that point, preferably with sizes, maybe someone finds a candidate compound. I guess all products must stay liquid (or solid?) at 200°C, not obvious. A transition between isomers, like quadricyclane does?
Can you explain why you need few µs if the heat must last 100ms?
I know this isn't exactly your query, but I'd consider non-chemical processes too.
Have a fast jet of gas like argon around 200°C or over. Introduce the powder near the nozzle, as is done for paint, waterjet cutting and so on. Blow the jet on a target surface. At 500m/s and 50mm, the powder resides only 100µs (adjustable) in the hot gas, so it may need >200°C. The impact of the hot PA12 microbeads sinters them. You may want a low pressure to the beads continue to the target when the gas deviates before the surface.
Make a suspension of the microbeads in a neutral liquid with a little controlled bit of solvent that softens the beads' surface and dissolves a small amount. Paint the target with the suspension. Evaporate the neutral liquid and the solvent.
Make a suspension of your microbeads in a metal liquid near room temperature but not aggressive to PA12, maybe gallium and its alloys. You need a high proportion of beads so the suspension is viscous enough. Paint the target. Put a short strong current in the paint, possibly by induction. Control the cooling time by the target's heat conduction. Remove the metal, maybe by capillarity using desoldering wick or similar.
Heat the PA12 by microwaves. It's a lossy material, but microseconds work against you here. Adding graphite nanopowder may help.
Sweep a light spot on the target. Concentrated light heats quickly and is quickly away. Depending on the layer thickness (is it a layer?) you can heat the target instead.